The present invention relates to a frictional spot joining method and apparatus in which a plurality of metal members are provided to lap over and these metal members are joined with a frictional heat by a rotating tool.
Recently, aluminum alloy or the like have been widely used as material to be applied to a vehicle body of an automotive vehicle for the purpose of a weight reduction. Accordingly, for example, joining of members made of aluminum alloy or joining of an aluminum-alloy member and a member made of iron or steel have been used. Herein, since joining with welding may be difficult for these members, joining with rivet has been generally used for these members. However, the rivet joining is generally expensive.
Meanwhile, a frictional spot joining is known as an appropriate joining method that can be applied with reasonably low costs. In this joining method, a plurality of metal members are provided to lap over, a rotating tool is pushed against the metal members such that the metal member is softened and made in a plastic flow state by a frictional heat, whereby the metal members can be joined in a solid state (joining in a slid state without melting) with a temperature lower than a melting point of a work to be joined (metal members).
For example, US Patent Application Publication No. 2005/0035180 A1 discloses a frictional spot joining method and apparatus for a zinc-plating steel plate and an aluminum plate. This apparatus comprises a joining device that integrally has a rotating tool, a receiving tool that is disposed to face the rotating tool on a rotational axis of the rotating tool, and a drive device operative to rotate and move the rotating tool in a pushing direction.
Meanwhile, a rotational runout of the rotating tool tends to occur when a tip (shoulder portion) of the rotating tool contacts a work in this frictional spot joining. Namely, a continuous movement of the receiving tool in a direction that is substantially perpendicular to the rotational axis of the rotating tool tends to occur due to a rapid increase of the contact resistance. It is generally known that a projection (pin) with a smaller diameter is formed at the shoulder portion in order to restrain the above-described rotational runout. Namely, the pin contacts first with a small contact resistance at the joining, thereby providing a proper centering (anchor function). The rotational runout can be restrained by this anchor function.
However, it may be difficult to prevent the rotational runout perfectly by this anchor function, so the rotational runout would exist to some extent. However, this rotational runout would cause an adhesion (sticking) of part of the work to the shoulder portion. This adhesion would deteriorate the above-described anchor function and promote the rotational runout. As a result, the rotational runout would occur more badly. This would deteriorate the joining quality or cause damages to the rotating tool, its drive mechanism or other tools.
In order to avoid the above-described situation, it would be effective to restrain the rotational runout (movement) of the rotating tool further, so an initial occurrence of the adhesion to the shoulder portion could be prevented. Japanese Patent Laid-Open Publication No. 2003-205374 discloses a frictional spot joining device equipped with a fixing device having a spring and a pressing member that are provided around the rotating tool, which is operative to restrain the rotational runout (corresponding to a lateral runout in this publication).
The pressing member of this fixing device is a substantially cylindrical member that is provided coaxially outside the rotating tool, which restrains via a bearing a movement of the rotating member in a direction that is perpendicular to the rotational axis of the rotating member. And, when the joining is executed, this member contacts the work first, prior to the rotating member contacting the work, and is pressed against the work by the spring. Thus, the work and pressing member are integrated temporarily, so the movement of the rotating tool in the direction perpendicular to the rotational axis relative to the work is restrained via the bearing. Namely, the rotational runout can be restrained.
However, the fixing device shown in the above-described latter publication comprises the spring, bearing and so on, therefore its structure would be rather complicated. Also, its size would be relatively large so as to enclose the rotating tool, so that the joining in a narrow space would be difficult. In fact, the rotating tool is required to go through a narrow gap space between clamping tools fixing the work and approach to the work, so a large-sized device would be inappropriate.